1. Field of the Invention
The present invention relates generally to radiation therapy planning for the treatment of tumors or for stereotactic radiosurgery and more particularly to the optimization of the radiation dose delivered to a patient by inverse treatment planning.
2. Description of the Background Art
Medical equipment for radiation therapy treats tumorous tissues with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high-energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to a treatment volume, which can be the tumor site or the site of radiosurgery. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the treatment volume.
The adverse effect of irradiating of healthy tissue may be reduced, while maintaining a given dose of radiation in the treatment volume, by projecting the external radiation beam into the patient at a variety of angles around a fixed axis, with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, are changed, reducing the total dose to each such element of healthy tissue during the entire treatment. The irradiation angles around the fixed axis are called gantry angles, since a gantry holding the radiation source and associated beam-delivery equipment rotates around the fixed axis.
As part of collimating the beam to the outline of the tumor, two offset angle of the radiation beam, with respect to a radius line between the radiation source and the center of rotation of the radiation source, may be adjusted to allow the treated volume to be other than at the center of rotation. The set of pencil beams thus created form a cone beam in three dimensions that can irradiate the complete treatment volume for each gantry angle. Changing the offset angles and the fluence for each of the pencil beams simultaneously allows treatment volumes having irregular cross-section within planes parallel to the plane of the gantry to be accurately targeted and radiation doses to healthy tissues to be minimized.
The objective of modern radiation therapy planning is to provide the values of a set of beam- delivery parameters that should be used in irradiating the treatment volume with adherence to a prescribed dose, while limiting the irradiation of healthy tissues to tolerable prescribed doses, lower than the dose to the treatment volume. Beam-delivery parameters may include gantry angles, the pencil beam offset angles within each selected gantry angle, and the fluence for each selected pencil beam.
The solution to the treatment planning problem is normally carried out as a direct problem: given a known set of beam characteristics and an initial estimate of the beam-delivery parameters, the dose deposited in a particular patient's tissues is calculated. This calculation can be made with accuracy in radiation treatment installations, using commercially-available computer programs or programs developed at the particular installation. If the dose distribution resulting from the initial estimate of the beam-delivery parameters satisfies the therapist's prescription reasonably well, it is then refined by trial-and-error. Although specific direct calculations can be very accurate, there is no assurance that the process will arrive at optimum beam-delivery parameters because of the trial-and-error nature of their selection.
The conformal treatment planning problem is an inverse problem in the sense that it inverts the calculation of dose distribution: Given a set of required doses, data on the patient's anatomy and the characteristics of all the available pencil beams, it calculates the beam-delivery parameters that best approximate the required doses. Of foremost importance in the inverse problem is the definition of the criterion that determines optimality. Current art can be summarized as either:
a) minimizing the sum of the squares of the errors between the dose desired in each volume element of the treatment volume and of sensitive tissues in the beam paths and the dose that would be delivered by the treatment plan (see G. Starkschall, "A constrained least-squares optimization method for external beam radiation therapy treatment planning", Med. Phys. 11 (5), pp. 659-665, 1984; T. W. Holmes, T. R. Mackie and P. Rechwerdt, "An iterative filtered backprojection inverse treatment planning algorithm for tomotheraphy", Int. J. Radiation Oncology, Physc., Vol. 32, No. 4, pp. 1215-1225, 1995; T. Borfeld and A. L. Boyer, "The exponential Radon transform and projection filtering in radiotheraphy planning", Int. J. Imaging Systems and Technology, Vol. 6, pp. 62-70, 1995; A. Gustafsson, B. K. Lind and A. Brahme, "A generalized pencil beams algorithm for optimization of radiation therapy", Med. Phys. 21 (3) pp. 343-356, 1994; T. Borfeld and W. Schlegel, "Optimization of beam orientations in radiation therapy: some theoretical considerations", Phys. Med. Bio. 38, pp. 291-304, 1993; U.S. Pat. No. 5,317,616 S. Swerdloff, T. R. Mackie and T. Holmes, "Method and apparatus for radiation therapy", May 31, 1994),
or b) minimizing some cost functions which are functions of the same errors indicated in a) (see S. Webb, "Optimizing radiation therapy inverse treatment planning using the simulated annealing technique", Int. J. Imaging Systems and Technology, Vol. 6, pp. 71-79, 1995),
or c) maximizing the smoothness of the dose distributions in the treatment volume and in the sensitive tissues while keeping the actual delivered doses near the prescribed doses (see W. A. Sandham, Y. Yuan and T. S. Durrani, "conformal therapy using maximum entropy optimization", Int. J. Imaging Systems and Technology, Vol. 6, pp. 80-90, 1995),
or d) solving a "feasibility" problem which results in a solution that satisfies some prescribed upper and/or lower bounds for the doses in the treatment volume and sensitive tissues (see W. D. Powlis, M. D. Altschuler, Y. Censor and E. L. Buhler, "Semi-automated radiotheraphy treatment planning with a mathematical model to satisfy treatment goals", Int. J. Radiation Oncology, Biol. Phys., Vol. 16, pp. 271-276, 1989),
or e) proposing the use of a method for solving a set of optimization functions, but not describing the behavior of the solutions in the presence of inconsistencies in the data (see U.S. Pat. No. 5,418,827, J. O. Deasy and R. De Leone, "Method for radiation therapy planning", May 23, 1995; U.S. Pat. No. 4,373,844, V. Smith and R. A. Stone, "Inverse treatment planning method and apparatus for stereotactic radiosurgery", Dec. 19, 1994; U.S. Pat. No. 5,205,289, T. L. Hardy, G. W. Glover, L. D. Breynildson, "Three-dimensional computer graphics simulation and computerized numerical optimization for dose delivery and treatment planning", Apr. 17, 1993; U.S. Patent 3,987,281, L. Hodes, "Method of radiation therapy treatment planning", Oct. 19, 1976),
or f) using the algebraic reconstruction technique (ART) to find the optimum set of beam fluences with minimum norm (see Y. Yuan, W. A. Sandham, T. S. Durrani, J. A. Mills and C. Deehan, "Application of Bayesian and maximum entropy optimisation to conformation radiotherapy treatment planning", Applied Sig. Process, 1, pp. 20-34, 1994.
All the above methods suffer from the fact that the laws that govern the loss of energy and dose deposition of photons (.gamma.- or x-rays) in matter are fixed and the doses or bounds prescribed by a therapist for the treatment volume and for the sensitive tissues based on medical knowledge will, in general, be inconsistent with one another from the point of view of the physical laws. As a consequence of that inconsistency, attempts by any of the above methods to yield a therapy plan that tries to satisfy all the specified doses will result in a more irregular dose to the treatment volume than desirable and, in some cases, lead to meaningless solutions. Irregularity in the dose delivered to the treatment volume may result in under-irradiation of some parts of a tumor, for example, which lowers the probability of tumor control.